I have two mains power inputs (in case one fails, the other is likely to still work), but the device only has one power supply, so I decided to use a relay to switch between the inputs (the device should be able to cope with a momentary interruption during the switching), like this: schematic diagram

In theory and for low voltages this should work just fine. When the primary input fails, the relay switches to the backup input. Is there any potential problem I should be aware of when using this for 230V?

Connected to this: is there a way to add a fuse to this circuit so if something (the relay or the device) shorts out that fuse blows instead of tripping the circuit breaker? Maybe some resistor to limit the short circuit current?


2 Answers 2


The relay coil must be for 230 V AC. Most relays are for voltages less than 24 V, and also DC. An AC relay will consume less power.

The contacts must be for 250 V AC minimum. But your demands are different than for a common 250 V contact. If there's a spark over an open contact in a normal serial switch application then no harm is done, because the circuit is designed to allow current through the contacts anyway. But that's not the case here: a spark between two contacts may cause a short circuit.

Also if the second power source is a different phase of the 230 V then you can have 400 V between the two contacts. It would be better then to have the neutral and the phase of V1 on one contact and the phase of V2 and the neutral to the other contact. So not the two "-" together, but the "-" of one to the "+" of the other one.

In any case I would recommend using a 500 V relay.

  • \$\begingroup\$ One more safety factor: if there is a fault between the NO and NC fixed contacts during a power failure, you will be back-feeding power to the mains supply, which can be a hazard to workers repairing the fault. So there may be more strict requirements on such a supply transfer switch. \$\endgroup\$
    – Evan
    May 13, 2017 at 17:37
  • \$\begingroup\$ this is why relays aren't used, for example, in motor reversal applications. What they use are mechanically interlocking contactors that guarantee the "break before make" condition. They are contactors that are mounted side to side, with a pin that sticks out when the contactor is activated, and it prevents the other contactor from being activated. \$\endgroup\$
    – hjf
    Jan 5, 2018 at 13:17

First, if you are asking these questions, I would get a qualified electrician involved. For your safety.

  1. Make sure the relay is rated for 230 V, and has margin past that.
  2. Make sure the relay is rated for the maximum current it will see.
  3. Make sure the relay isn't a latching one. Those stay in a state when they loose power on the coil.
  4. In response to comment below, make sure it is "break before make" to avoid shorts.
  • 1
    \$\begingroup\$ Also make sure of the relay coil voltage. \$\endgroup\$ Dec 15, 2012 at 5:42
  • \$\begingroup\$ Well, the datasheet specifies 250v (maximum switching voltage 400V, dielectric strength between open contacts 1kV) and 400VA load at 230VAC so it should be OK. However, how likely is it that the relay would short NC to NO (and thus short both inputs together)? Also, is there a way to make sure that in the event that the relay shorts out, it trips some local protection device, but not the whole circuit breaker? With a normal fuse in the case of a dead short the circuit breaker trips in addition to the fuse blowing. \$\endgroup\$
    – Pentium100
    Dec 15, 2012 at 8:34
  • \$\begingroup\$ To break fuse and not home circuit breaker, the fuse must be fast and significantly less Amp rated, and breaker must be slow and higher Amp rated. In normal case 1 : 2.5 Amp ratio is usually accepted when using two same (i.e. fast) fuses/breakers. So like 6:16 A or 10:25 A should be OK in most cases. But it isn't 100%. Using a "slow" home circuit breaker helps in the case of overload, but doesn't provide much help in case of a total shortcut. \$\endgroup\$
    – Al Kepp
    Jan 16, 2013 at 12:04

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